Tom, that's a neat spring, and I can see how it would work beautifully. The oval pierced spring is the only one that I have seen, and is the one similar to what Nicholas Hawk used.
Here's a sketch of the action that I think I might make for my rendition. I'm open for critique that will improve the design. Bear in mind that my rifle will have hooked breeches - I MUST remove my barrels for cleaning.
I have a double pin design that is too complex, 2 springs, pins connected with a bar. The piece that retracts them is not attached to bar but is simply under it and levers the bar back. It seems to work well and all the holes are as reamed since the pins do not have to tilt as they move. It has seen quite a bit of shooting and its still tight.
The one in the works will have a single circular spring, 2 pins like Taylors and a lever as used here.
I use one of those flintlock cleaning gizmos that TOW sells on this rifle works about as well as cleaning a hooked breech in a bucket.
This requires removing the pans. PITA
The breeches are patent breeches that are attached with screws through the plate, 1/4 x28.
The plates are 5/16.
Pivot is made from a 3/8" grade 8 bolt.
Forend panels are 2 pieces.
Current project will have round holes in the plates and the barrels will be held to it with taper pins. At least this is the current plan. The pins will also allow removing the breeches which on this rifle will be "plain".
Round holes in the plates are far easier to do than square and I can turn the breeches to fit on the lathe.
Any looseness in the barrel/plate joint will likely affect accuracy. This is true of single hooked breeches and I cannot see it not affecting a swivel breech.
I am currently attempting to make dies for brass forend panels.
I have a 48"x 12" x .040? brass sheet from Alcobra Metals to mangle in the process.
Dan